This invention was made in the course of, or under, a contract with the United States Department of Energy. It relates in general to preparation of graphite and more particularly to the preparation of graphite having extremely high thermal stress resistance. Graphite is a porous material with two defect distributions to consider in the continuum mechanical approach to fracture. The first defect is the large pore associated with particle boundaries. The second defect is the microporosity which extends on a fine scale distributed within the particulate and crystallites. The generally accepted model is that the large pores initiate a crack that propagates from pore to pore normal to the stress axis and constitute the critical defect. Microcracking, however, begins well below the failure stress and increases in number with increasing applied stress. The ability of graphite to achieve large strains to failure is limited by the ability of the structure to accommodate microcracking without resulting in rapid crack propagation to failure. This microcracking is actually a stress relief system which distinguishes graphite from classically brittle ceramic materials.
In the prior art manufacture of graphite, e.g. electrodes, a coke of low sulfur content is generally the preferred raw material. Low sulfur coke is used to reduce the sudden irreversible expansion upon heating to graphitization. This phenomenon results from evolution of sulfur-containing gases from the coke, and is known as "puffing." A number of methods have been used in the prior art to prevent the puffing, for example, adding a puffing inhibitor such as an iron or calcium compound in combination with TiO.sub.2 or ZrO.sub.2, disclosed in U.S. Pat. No. 3,563,705. Puffing generally occurs only in thermoplastic pitches containing sulfur. In thermosetting materials, such as bitumen and thermosetting pitches, the presence of sulfur does not result in significant puffing and increases the coefficient of thermal expansion; see, for example, U.K. Pat. No. 1,117,606 issued to the United Kingdom Atomic Energy Authority, June 9, 1968. An increased coefficient of thermal expansion is undesirable in high-temperature applications where dimensional stability is important.